This invention relates to a method of depositing a layer on an exposed surface of an insulating layer of material.
It has been known for some time that the grain structure of a deposited layer can be affected by the structure of the layer on to which it is deposited. This relationship is discussed in terms of aluminium layers deposited on titanium in U.S. Pat. No. 5,523,259 and on titanium nitride layers in U.S. Pat. No. 5,242,860. Perhaps one of the most complete and recent expositions on the state of the art concerning the grain structure of metal conductors on barrier layers and how a preferential grain structure is achieved is contained in WO 99/10921. However, there is no indication in the prior art of the relationship between the structure of metallic deposited layers and insulating layers upon which they lie. Further prior art gives no indication of how such an insulating layer may be treated to improve the deposited layer structure for these purposes.
In addition to bulk interconnects another technology in which this is significant is the formation of acoustic wave devices wherein the orientation of the piezoelectric layer can be significant in the performance of the device.
Thus in a first aspect the invention consists in a method of depositing a metallic layer or layers on the exposed surface of a previously deposited insulating layer upon a substrate including treating the exposed surface with hydrogen or a gaseous source of hydrogen in the presence of a plasma prior to the deposition of the metallic layer or layers.
Surprisingly it has been found that the exposure to hydrogen changes the structure of at least the exposed surface of the insulating layer in a sense to improve the orientation of a metallic layer and in particular a piezo electric layer subsequently deposited upon the substrate. This may be because hydrogen is implanted in the exposed surface or because the hydrogen modifies e.g. by etching the exposed surface or a combination of the two.
It is preferred that the extent of the hydrogen treatment is such that the Full Wave Half Maximum (FWHM) of the rocking curve on a preselected crystallographic plane of a deposited layer is less than 2.5xc2x0.
The plasma may be an Inductively Coupled Plasma in which case the substrate may be placed on an RF biased platen, which may be heated. Alternatively the plasma process may be Reactive Ion Etching. In the first case the process time for the hydrogen treatment may be between 35 and 25 minutes, and in the second case the treatment period may be more than 5 minutes and less than 15 minutes.
Typically the substrate will be a semiconductor such as silicon or the insulating layer will be silicon dioxide. Where the process is being used in the form of an acoustic wave device, a deposited layer will be preferably required to have a narrow x ray diffraction peak half width on (002) to function as a piezo electric thin film. This deposited layer is preferably aluminium nitride. It is preferred that the aluminium nitride is deposited at a temperature below 500xc2x0 C.
As is known in the art the FWHM rocking curve of a diffraction peak is a good indication of degree of orientation. This rocking curve is obtained by rotating a sample in an x-ray beam, which is directed at the surface being inspected. At a particular angle the curve produces a reflectance peak and by rocking the sample about that peak it is possible to determine the angle of rock needed to move the sample from half the maximum intensity on one side of the peak to the corresponding point on the other side of the peak. This angle is referred to as the FWHM measurement and the narrower the angle the better ordered the structure.
In an experiment aluminium nitride was deposited onto an underlayer of aluminium (that forms one electrode) in turn deposited upon a titanium adhesion layer upon an insulting layer of silicon dioxide. The silicon dioxide had been treated in one of three ways and the FWH rocking curve of the aluminium nitride measurement was obtained on (002).